It is photoresist technology and materials that have been conventionally used for forming patterns of conductive, semi-conductive, or insulating materials on a substrate in fabrication of devices such as electronic components, integrated circuits, small-scale photonic components, printed circuit boards, and interconnecting components for such devices. Using conventional photoresist materials and methods, a photoresist layer is disposed and patterned onto a substrate or onto other patterned materials so that it either protects or exposes an underlying material for an etching process that follows. The photoresist layer is normally formed of a polymeric, organic material that is substantially unaffected by a metal deposition and/or metal removal processes and, accordingly, is fit to protect underlying areas from etching processes. A pattern is formed by exposing the photoresist material to actinic radiation through an appropriate mask (such as a glass master, for example, often used in photolithographic techniques) or by directly writing of a pattern with an electronic beam. In the first scenario, the incident radiation (such as the UV or X-ray radiation, for example) causes a chemical reaction in the photoresist material, and modifies its relative solubility. The process that follows includes dissolving and removing the unexposed areas, for example, of the photoresist while leaving the exposed portions intact to transfer the pattern towards the underlying substrate.
Conventional UV photoresist etching is a relatively costly process, requiring substantially high-energy radiation sources to drive the needed chemical reactions. Preparation of masks and photo-tools can be very expensive, error-prone, and time-consuming. The use of masking techniques is necessarily limiting the achievable spatial resolution and places considerable demands on the design of supporting optical components. Moreover, conventional photoresist materials behave linearly with respect to subjected dosages (or energy per volume). That is, in the case of a traditional resist material, the overall exposure that is used to create the pattern in the resist material is a sum of the net individual exposures. The final pattern can be determined by the sum of the individual photonic events that create the chemical changes in the traditional material, for example acid site generation and cross linking. This makes traditional resist materials less desirable for technologies that require a sequence of multiple exposures to generate sub-resolution patterns since the traditional resist needs to be exposed, developed and appropriately removed to make visible the first pattern before any additional pattern can be formed on the same material. And the cause of it is, of course, that the traditional resist material permanently records any previous photonic event (interaction with incident light to which such material is sensitive).
In attempts to improve upon the expense and complexity of conventional photoresist etching, a number of alternative fabrication techniques have been adapted. Examples of such alternative methodologies include ablation methods (now recognized to be a poor performer for complex patterning situations that require multiple layers due to leaving hard-to-remove debris on the patterned substrate) and a transfer mask (the shortcoming of the use of which arises from a need to use standoffs causing the loss of spatial resolution of the transferred pattern).
Another alternative to using photoresists that is being researched includes the use of thermal resist materials (and, in particular, non-linear metallic thermal resists). Unlike photoresist substances that undergo chemical changes in reaction to light of high-energy, thermal resist materials undergo chemical or physical reactions in response to heat energy. In general, thermal resist materials are advantageous over photoresists because of their non-linear behavior, simpler chemistry, lower cost, and relative insensitivity to ambient light. Furthermore, in comparison with the traditional resist materials, the use of a non-linear thermal resist is advantageous for the use in multiple exposure lithography. Specifically, unlike the traditional resist material, the non-linear metallic thermal resist does not “remember” prior exposure to light unless it was heated above the eutectic temperature during such prior exposure. As a result, a spatial region or area of the non-linear thermal resist material that has not reached the eutectic temperature reverts to its original material state after the exposure to light is over.
Despite these inherent advantages over photoresists, a number of practical considerations remain before the use of thermal resists becomes versatile. In particular, related art and practical limitations clearly indicate that currently considered non-linear thermal resists lack the sensitivity to activating heat-causing radiation that is necessary for use in multiple-exposure lithography.
U.S. Pat. No. 7,989,146 (Burberry et al.) is but one example that discloses a method for component fabrication using thermal resist materials but do not address the issue of controlling and improving the sensitivity of the deposited thermal resist films. The described non-linear resists described are not applicable to conventional and commercial high-NA scanner pulse irradiances due to their radiation insensitivity. The typical energy flux values required to produce the reactions in the non-linear thermal resist described by Burberry et al. are between 300 mJ/cm2 per pulse and 500 mJ/cm2 per pulse, which values are about 30 to 50 times larger than commercially available high-resolution scanner systems are structured to deliver to the substrate.
While attempts to improve sensitivity of bi-metallic thin-film resists were made (for example, Tu et al. in “Inorganic Bi/In Thermal Resist as a High-Etch Ratio Patterning Layer for CF4/CHF3/O2 Plasma Etch,” Proc. of SPIE Vol. 5376, pp. 867-878, 2007 considered varying the thickness of the metallic film), no practical solution to the problem of decreasing the effective heat of transformation required for a metallic thermal thin-film resist to form an alloy. In particular, sensitivity levels sufficient for use of metallic thin-film resists to conventional pulse irradiances from lithography scanners have not been achieved.
The present invention overcomes practical limitations that have been preventing the use of the existing metallic thin-film resists in multi-exposure lithography by providing a new class of higher sensitivity non-linear optimized multi-layer thin-film thermal stacks that enable processes of multi-patterning with pulse irradiances that are less than about 10 mJ/cm2 for use with the now available on conventional high-NA immersion tools, to prevent damaging imaging optical materials and coatings on the substrate that would occur at higher levels of irradiance. These results are achieved by principal, deliberate restructuring the metallic thermal thin-film resists through the use of multi-layer films, material dopants and/or the application of thin-film stresses to decrease the effective heat of material transformation in thermal resists.